In the development of components of zirconium, such as in the formation of nuclear fuel cladding for use in containing fuel in a pressurized water reactor or boiling water reactor, the zirconium or zirconium alloy articles must be carefully formed to detailed specifications. In formation of nuclear fuel clad tubing, for example, an initial zirconium alloy tube is pilgered a number of times to reduce the size thereof and provide properties and sizes to specifications. A significant part of the formation of such clad tubing is the etching of the tube to remove defects from the tubing surface, especially the inside surface, which will confront the nuclear fuel, and also to increase the inside diameter of the clad tubing to specified dimensions. Such etching steps are generally used after each of three pilgering stages and twice after the final pilger mill pass. Especially useful zirconium alloys used in formation of nuclear fuel cladding and other components of nuclear reactors are those known as Zircaloy -2 and Zircaloy -4. Zircaloy -2 contains, by weight, about 1.2 to 1.7 percent tin, 0.07 to 0.20 percent iron, 0.05 to 0.15 percent chromium, and about 0.03 to 0.08 percent nickel, the balance being zirconium, while Zircaloy -4 contains, by weight, about 1.2 to 1.7 percent tin, 0.12 to 0.18 percent iron, and 0.05 to 0.15 percent chromium, the balance being zirconium.
In the etching of zirconium or zirconium alloy articles, it is known to use aqueous hydrofluoric acid-nitric acid baths. In the etching of tubes, for example, tubes are immersed in an aqueous bath containing hydrofluoric acid, preferably in an amount by weight of 3 percent, and nitric acid, preferably in an amount by weight of 15 percent, until the required surface cleaning and polishing of the article is obtained. Etch rates of the baths decrease with use until a limiting rate of about 20 percent of the fresh or initial bath rate is reached. At this stage the spent baths, which generally contain about 24 g/l of dissolved zirconium alloy, are discarded. The spent etching baths must then be treated to render them disposable and the baths discarded, an expensive procedure. The spent baths contain, among other components, various zirconium compounds or complexes, some tin components, when Zircaloys are etched, residual hydrofluoric acid and residual nitric acid.
Various attempts have previously been made to regenerate or replenish hydrofluoric acid-nitric acid baths used in treating zirconium articles. In U.S. Pat. No. 4,105,469, a pickle acid bath for cleaning zirconium is generated by adding sodium fluoride to a spent hydrofluoric acid-nitric acid pickle liquor to precipitate zirconium fluoride therefrom. The addition of the sodium fluoride is measured to precipitate sodium hexafluoro zirconate to produce a pickle liquor containing from 3-7 grams zirconium per liter. Hydrofluoric acid is added to make up for the amount of acid used in pickling and, when necessary, nitric acid is added to bring the solution up to the pickling concentrations. The spent pickle liquor is removed from pickling tanks and the sodium fluoride added in separate tanks, with the acids also added in separate tanks. In U.S. Pat. No. 3,048,503, titanium or zirconium sheets are pickled by introducing them into a circulating body of aqueous pickle liquor containing 2-4 percent hydrofluoric acid and 15-30 percent nitric acid. The sheets are passed countercurrent to a flow of the pickle liquor, with partially spent pickle liquor withdrawn, cooled to precipitate metal values, metal values separated, and the hydrofluoric acid-nitric acid concentration of the spent pickle liquor adjusted, with the liquor then returned to the bath. German patent disclosure No. 2828547 describes a process for controlling the composition of a pickling bath for zirconium where a partial volume of the bath is withdrawn, the metal in the partial volume precipitated to form a difficult to dissolve compound, and the concentration of the compound determined in dilution by turbidity measurement. The bath is then regenerated by adding fresh hydrofluoric acid-nitric acid solutions to the bath while a like volume of used pickle liquor is drawn off from the bath.
It is an object of the present invention to regenerate an aqueous hydrofluoric acid-nitric acid etching bath for zirconium metallic articles so as to extend the life of said bath, while achieving acceptable etching rates.
It is another object of the present invention to regenerate an aqueous hydrofluoric acid-nitric acid etching bath for zirconium metallic articles without the need to precipitate and remove dissolved zirconium material from said bath.